This invention relates to an improved method of joining two or more ceramic shapes to form a unitary ceramic structure. More specifically the invention relates to a method of joining two or more shapes of ceramic composite material to form a unitary structure. Still more specifically this invention relates to a method of joining or bonding two or more shapes of silicon carbide composite material to form a unitary structure without degrading the mechanical properties of the joined material while providing a joint capable of withstanding temper-atures up to about 1000.degree. C.
There is considerable interest in the development of advanced ceramic fiber/ceramic matrix composites for high temperature applications. One such composite is a silicon carbide fiber-reinforced silicon carbide which is under development for structural applications at temperatures up to 1000.degree. C. This composite may contain about 40 vol. % silicon fibers, and is infiltrated to about 85% of theoretical density with silicon carbide. For this material, reductions in strength have been shown to result from prolonged exposure to elevated temperatures. For example, strength losses have been observed above approximately 1000.degree. C., while greater losses occurred above 1200.degree. C. This reduction in properties has been attributed to degradation of the SiC fibers that occurs at these temperatures. However, strength losses can be minimized to temperatures approaching 1400.degree. C. by coating the fibers with carbon and externally coating the composites with a protective layer of SiC. In order to fully realize the potential of these composites, reliable and practical joining techniques must be developed that will permit the use of these materials as complex shaped components or allow their integration into existing engineering designs. Such joints require strength and toughness comparable to the joined material under the temperature and environmental conditions of the projected application
There exists a number of methods for joining two or more shapes of ceramic material such as silicon carbide. These techniques include direct diffusion bonding, codensification of interlayer and green bodies, diffusion welding or brazing with boride, carbide and silicide interlayers, hot pressing of sinterable SiC powder, bonding with polymer precursors, brazing with oxide, or oxynitride materials, reactive metal bonding, and active metal brazing However, none of these techniques, except active metal brazing are suitable for use with the composite ceramics such as the silicon carbide fiber-reinforced silicon carbide matrix composites (SiC/SiC) because they generally require temperatures above about 1650.degree. C. Active metal brazing, while operable at temperatures low enough to join the composite shapes without harming the composite material, results in joints which are unable to withstand the temperatures at which the composite material is capable of operating, so that the joined structure cannot be used at its full temperature potential.
What is needed is a method for joining shapes of ceramic composite materials and most particularly shapes of silicon carbide composites, which is capable of joining the shapes at temperatures no higher than about 1450.degree. C. while providing a joint which is capable of withstanding the thermal and mechanical conditions of which the composite material is capable.